1. The Field of the Invention
The invention relates to an alkaline earth metal aluminosilicate glass for light bulbs, the light bulbs having molybdenum parts, in particular for light bulbs having bulb temperatures of more than 700° C., and also its use.
2. The Description of the Related Art
Glasses for high-temperature lamps, which generally mean lamps having bulb temperatures from 550° C. upwards, must meet demanding requirements.
Possible glasses are alkaline earth metal alumino(boro)silicate glasses. The glasses must be essentially free of alkali metal oxides, since alkali metal ions interfere in the regenerative halogen cycle of the lamp. This is because during operation of the lamp, an equilibrium is established between formation and decomposition of tungsten halides due to reaction of the tungsten vapor from the filament and the halogen/inert gas mixture. The decomposition reaction takes place at higher temperatures than the formation reaction, so that the tungsten re-deposits on the filament. If this cycle is disrupted by contaminating components, such as alkali metal ions, the tungsten deposits not on the filament but on the inside of the glass bulb as a black shiny interfering coating.
For use as light bulb glass for light bulbs that have molybdenum parts like electrodes or leads, the thermal expansion of the glass has to be matched to molybdenum so that gas-tight stress-free fusion of metal and glass is achieved.
A further requirement which a glass suitable for use as bulb glass for light bulbs must meet is suitability for the drawing of tubes. For this purpose, it must be sufficiently stable to crystallization.
Further requirements which the glass must meet are a high glass transition temperature Tg and a low processing temperature VA. A high glass transition temperature has the advantage that the glass is able to withstand a high thermal stress. A low processing temperature has the advantage that fusion can be carried out in a high yield with a relatively low energy input and the glass can be processed further economically. However, these properties usually run counter to one another.
DE 37 36 887 C2 describes B2O3-free low-CaO glasses. These glasses have the disadvantage of high processing temperatures.
Glasses requiring B2O3 are also known for incandescent light bulbs:
Thus, the glasses for fusion with molybdenum known from U.S. Pat. No. 3,310,413 contain from 4 to 9 percent by weight of B2O3. The sealing and bulb glasses known from DE 33 05 587 A1 also require from 3 to 7 percent by weight of B2O3 and also high proportions of BaO (from 11 to 16 percent by weight). Such high contents of B2O3, especially in combination with MgO, reduce the viscosity values so that these glasses are not suitable for halogen lamps having bulb temperatures of more than 650° C., for example about 700° C. The low thermal stressability of the glasses leads to bulging of the light bulb which can proceed so far that the bulb explodes. An example of such a glass is the commercially available glass V0 having the composition (in percent by weight): 56.8, SiO2; 16.4, Al2O3; 4.7, B2O3; 5.8, MgO; 7.8, CaO; and 8.0, BaO and having an upper cooling temperature UCP of 721° C.
The glasses known from DE 197 58 481 C1, DE 197 47 355 C1, DE 100 63 05 A1 and DE 10 2004 048 097 A1 also contain B2O3. They contain relatively little CaO and relatively little Al2O3. They do not simultaneously meet the requirements with respect of Tg and VA. The same applies to the glasses known from DE 100 22 769 A which contain relatively little CaO.
Furthermore, a low melting point of the glass is desirable.